Non-Dispersive infrared (NDIR) gas analyzers have been used for detecting the presence and concentration of various gases for over four decades. The NDIR technique has long been considered as one of the best methods for gas measurement. In addition to being highly specific, NDIR gas analyzers are also very sensitive, stable and easy to operate and maintain.
In contrast to NDIR gas sensors, the majority of other types of gas sensors today are in principle interactive. Interactive gas sensors are less reliable, generally nonspecific, and in some cases can be poisoned or saturated into a nonfunctional or irrecoverable state.
Despite the fact that interactive gas sensors are mostly unreliable and that the NDIR gas measurement technique is one the of best there is, NDIR gas analyzers have still not enjoyed widespread usage to date mainly because of the fact that their cost is still not low enough as compared to other inferior gas sensors for many applications.
In the past, NDIR gas analyzers typically included an infrared source, a motor-driven mechanical chopper to modulate the source, a pump to push or pull gas through a sample chamber, a narrow bandpass interference filter, a sensitive infrared detector plus expensive infrared optics and windows to focus the infrared energy from the source to the detector. In an attempt to reduce the cost and simplify the implementation of the NDIR methodology, a low-cost NDIR gas sensor technique was earlier developed. This low-cost NDIR technique employs a diffusion-type gas sample chamber of the type disclosed in U.S. Pat. No. 5,163,332, issued on Nov. 17, 1992 to Wong, the present applicant. This diffusion-type gas sample chamber eliminates the need for expensive optics, mechanical choppers and a pump for pushing or pulling the gas into the sample chamber. As a result, a number of applications using NDIR gas sampling technique, which were previously considered impractical because of cost and complexity, have been rendered viable ever since.
In the ensuing years since U.S. Pat. No. 5,163,332 was issued, Wong, the present applicant, has continued to refine and improve low-cost NDIR gas sampling techniques as evidenced by the issuance of U.S. Pat. No. 5,222,389 (June 1993), U.S. Pat. No. 5,341,214 (August 1994), U.S. Pat. No. 5,347,474 (September 1994), U.S. Pat. No. 5,453,621 (September 1995), U.S. Pat. No. 5,502,308 (March 1996), U.S. Pat. No. 5,747,808 (May 1998), U.S. Pat. No. 5,834,777 (November 1998) and U.S. Pat. No. 6,237,575 (May 2001) to same. Until recently, efforts to reduce the cost of an NDIR gas sensor have been concentrated mainly in the areas of developing lower cost infrared components, improving sensor structural and optical designs and forging innovations and simplifications in electronic signal processing circuits. Hardly any significant effort has been devoted to sensor cost reduction via new NDIR sensor methodology.
Up until now, the most prevalent NDIR gas sensor today is a dual beam device having a signal and a reference beam implemented with a single infrared source and two separate infrared detectors, each having a different interference filter. The signal filter contains a narrow spectral passband that allows radiation relevant to the absorption of the gas to be detected to pass. Thus the presence of the gas of interest will modulate the signal beam. The reference filter contains a narrow spectral passband that is irrelevant to the gas in question and also to all the common gases present in the atmosphere. Therefore the reference beam will stay constant and act as a reference for the detection of the designed gas species over time. Although the dual beam technique works well for a host of applications, especially with the detection of relatively low concentration of Carbon Dioxide (CO2) gas (400-2,000 ppm) for HVAC (Heating, Ventilation and Air Conditioning) and IAQ (Indoor Air Quality) applications, the cost of the sensor is limited by the expensive detector package which contains two detectors each equipped with a different interference filter. Furthermore, the dual beam NDIR gas sensor still has a number of shortcomings that require special treatments in order to render the sensor adequately reliable and stable for use over time. These shortcomings include the aging of the infrared source which might cause the spatial distribution of infrared radiation reaching the detectors to change; the same applies to the non-uniform aging of the inner reflective surfaces of the sample chamber affecting the spatial distribution of the impinging radiation at the detector assembly, the different aging characteristics for the two interference filters each being manufactured via different deposition processing steps and optical materials and finally the potential different aging characteristics for the two detectors.
Logic would dictate that in order to improve the performance and to lower the cost of the ever more popular dual beam NDIR gas sensor, one has to resort to system structural or optical simplification and/or system components reduction. Taking the case of the dual beam NDIR gas sensor as an example, there are two ways that one can accomplish these objectives. First, one can reduce the number of detectors from two to one which automatically implies that the number of interference filters would also be reduced from two to one. In other words, one can attempt to convert the dual beam sensor methodology to a single beam one. Alternatively, one can increase the measurement capability of the dual beam sensor from being able to detect just one gas into one that can simultaneously detect two or more gases. If either of these two cases is successful, it would be equivalent to being able to reduce the unit cost for the dual beam NDIR sensors.
It is of interest to note that back in 1991 and prior to the issuance of U.S. Pat. No. 5,163,332 (1992) to Wong for the advent of the so-called “waveguide sample chamber,” the same inventor has earlier advanced the concept of a single beam NDIR sensor methodology using a spectral ratioing technique with a differential temperature source in U.S. Pat. No. 5,026,992 (1991). After almost 15 years, this concept has to date neither been proven to be viable in theory nor has it been experimentally demonstrated in order to illustrate its practicality. It was found out only very recently by Wong, the current applicant and the original author of U.S. Pat. No. 5,026,992 (1991), that although the concept advanced in said patent was sound, the method did not work when the prescribed steps were followed exactly according to the teaching of the patent. In a companion patent application entitled “Ultra Low Cost NDIR Gas Sensors” and co-authored with C. W. Tse, filed Aug. —, 2005 with Attorney Docket No. 35.121, the disclosure of which is specifically incorporated herein by reference, the authors reported their experimental results in proving definitively the impracticality of implementing such a single beam NDIR sensor using differential source temperature technique as advanced in U.S. Pat. No. 5,026,992 (1991). The authors went on to advance a novel real time programmable infrared source control method which makes the single beam NDIR sensor concept as disclosed in U.S. Pat. No. 5,026,992 (1991) experimentally viable in practice. Furthermore, the same authors advance in the same disclosure a new single beam NDIR sensor methodology which can work with a non-genuine blackbody source such as a low cost miniature incandescent light bulb in lieu of an expensive genuine blackbody source as stipulated in U.S. Pat. No. 5,026,992 (1991).
There is still a long felt need in many industries and applications to use still lower cost NDIR gas sensors. It is this need that drives the current applicant to continue to develop new and novel sensor techniques in order to bring about NDIR gas sensors with the lowest possible costs. Based upon the latest experimental results the current applicant obtained very recently about U.S. Pat. No. 5,026,992 and additional related research and development work he has carried out, the concept disclosed in the referred to patent was revisited and has now been advanced into a new and experimentally valid framework such that a single beam NDIR gas sensor can actually be used, for the first time, to simultaneously detect two or more gas species in accordance with the teachings of this present application.